pipe_write contains a wakeup storm, 2 writers that write into the same
fifo can wake each other up, and spend 100% cpu time with wakeup/schedule,
without making any progress.
The patch below was part of -dj, and IIRC even of -ac, for a few months,
but it's more a rewrite of pipe_read and _write.
Could you merge it, or do you want a minimal fix?
The only regression I'm aware of is that
$ dd if=/dev/zero | grep not_there
will fail due to OOM, because grep does something like
for(;;) {
rlen = read(fd, buf, len);
if (rlen == len) {
len *= 2;
buf = realloc(buf, len);
}
}
if it operates on pipes, and due to the improved syscall merging, read
will always return the maximum possible amount of data. But that's a grep
bug, not a kernel problem.
The diff is older, but applies to 2.5.40 without offsets.
-- Manfred <<<<<<<<<<<<<<diff -urpN --exclude-from=/home/davej/.exclude bk-linus/fs/pipe.c linux-2.5/fs/pipe.c --- bk-linus/fs/pipe.c 2002-08-20 13:15:16.000000000 +0100 +++ linux-2.5/fs/pipe.c 2002-08-20 13:25:24.000000000 +0100 @@ -25,6 +25,9 @@ * * FIFOs and Pipes now generate SIGIO for both readers and writers. * -- Jeremy Elson <jelson@circlemud.org> 2001-08-16 + * + * pipe_read & write cleanup + * -- Manfred Spraul <manfred@colorfullife.com> 2002-05-09 */ /* Drop the inode semaphore and wait for a pipe event, atomically */ @@ -44,97 +47,81 @@ static ssize_t pipe_read(struct file *filp, char *buf, size_t count, loff_t *ppos) { struct inode *inode = filp->f_dentry->d_inode; - ssize_t size, read, ret; + int do_wakeup; + ssize_t ret; - /* Seeks are not allowed on pipes. */ - ret = -ESPIPE; - read = 0; - if (ppos != &filp->f_pos) - goto out_nolock; + /* pread is not allowed on pipes. */ + if (unlikely(ppos != &filp->f_pos)) + return -ESPIPE; + + /* Null read succeeds. */ + if (unlikely(count == 0)) + return 0; - /* Always return 0 on null read. */ + do_wakeup = 0; ret = 0; - if (count == 0) - goto out_nolock; + down(PIPE_SEM(*inode)); + for (;;) { + int size = PIPE_LEN(*inode); + if (size) { + char *pipebuf = PIPE_BASE(*inode) + PIPE_START(*inode); + ssize_t chars = PIPE_MAX_RCHUNK(*inode); - /* Get the pipe semaphore */ - ret = -ERESTARTSYS; - if (down_interruptible(PIPE_SEM(*inode))) - goto out_nolock; - - if (PIPE_EMPTY(*inode)) { -do_more_read: - ret = 0; - if (!PIPE_WRITERS(*inode)) - goto out; + if (chars > count) + chars = count; + if (chars > size) + chars = size; - ret = -EAGAIN; - if (filp->f_flags & O_NONBLOCK) - goto out; - - for (;;) { - PIPE_WAITING_READERS(*inode)++; - pipe_wait(inode); - PIPE_WAITING_READERS(*inode)--; - ret = -ERESTARTSYS; - if (signal_pending(current)) - goto out; - ret = 0; - if (!PIPE_EMPTY(*inode)) + if (copy_to_user(buf, pipebuf, chars)) { + if (!ret) ret = -EFAULT; break; - if (!PIPE_WRITERS(*inode)) - goto out; - } - } + } + ret += chars; - /* Read what data is available. */ - ret = -EFAULT; - while (count > 0 && (size = PIPE_LEN(*inode))) { - char *pipebuf = PIPE_BASE(*inode) + PIPE_START(*inode); - ssize_t chars = PIPE_MAX_RCHUNK(*inode); - - if (chars > count) - chars = count; - if (chars > size) - chars = size; - - if (copy_to_user(buf, pipebuf, chars)) - goto out; - - read += chars; - PIPE_START(*inode) += chars; - PIPE_START(*inode) &= (PIPE_SIZE - 1); - PIPE_LEN(*inode) -= chars; - count -= chars; - buf += chars; + PIPE_START(*inode) += chars; + PIPE_START(*inode) &= (PIPE_SIZE - 1); + PIPE_LEN(*inode) -= chars; + count -= chars; + buf += chars; + do_wakeup = 1; + } + if (!count) + break; /* common path: read succeeded */ + if (PIPE_LEN(*inode)) /* test for cyclic buffers */ + continue; + if (!PIPE_WRITERS(*inode)) + break; + if (!PIPE_WAITING_WRITERS(*inode)) { + /* syscall merging: Usually we must not sleep + * if O_NONBLOCK is set, or if we got some data. + * But if a writer sleeps in kernel space, then + * we can wait for that data without violating POSIX. + */ + if (ret) + break; + if (filp->f_flags & O_NONBLOCK) { + ret = -EAGAIN; + break; + } + } + if (signal_pending(current)) { + if (!ret) ret = -ERESTARTSYS; + break; + } + if (do_wakeup) { + wake_up_interruptible(PIPE_WAIT(*inode)); + kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT); + } + pipe_wait(inode); } - - /* Cache behaviour optimization */ - if (!PIPE_LEN(*inode)) - PIPE_START(*inode) = 0; - - if (count && PIPE_WAITING_WRITERS(*inode) && !(filp->f_flags & O_NONBLOCK)) { - /* - * We know that we are going to sleep: signal - * writers synchronously that there is more - * room. - */ + up(PIPE_SEM(*inode)); + /* Signal writers asynchronously that there is more room. */ + if (do_wakeup) { wake_up_interruptible_sync(PIPE_WAIT(*inode)); - kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT); - if (!PIPE_EMPTY(*inode)) - BUG(); - goto do_more_read; + kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT); } - /* Signal writers asynchronously that there is more room. */ - wake_up_interruptible(PIPE_WAIT(*inode)); - kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT); - - ret = read; -out: - up(PIPE_SEM(*inode)); -out_nolock: - if (read) - ret = read; + if (ret > 0) + UPDATE_ATIME(inode); return ret; } @@ -142,116 +129,90 @@ static ssize_t pipe_write(struct file *filp, const char *buf, size_t count, loff_t *ppos) { struct inode *inode = filp->f_dentry->d_inode; - ssize_t free, written, ret; + ssize_t ret; + size_t min; + int do_wakeup; + + /* pwrite is not allowed on pipes. */ + if (unlikely(ppos != &filp->f_pos)) + return -ESPIPE; + + /* Null write succeeds. */ + if (unlikely(count == 0)) + return 0; - /* Seeks are not allowed on pipes. */ - ret = -ESPIPE; - written = 0; - if (ppos != &filp->f_pos) - goto out_nolock; - - /* Null write succeeds. */ + do_wakeup = 0; ret = 0; - if (count == 0) - goto out_nolock; - - ret = -ERESTARTSYS; - if (down_interruptible(PIPE_SEM(*inode))) - goto out_nolock; - - /* No readers yields SIGPIPE. */ - if (!PIPE_READERS(*inode)) - goto sigpipe; - - /* If count <= PIPE_BUF, we have to make it atomic. */ - free = (count <= PIPE_BUF ? count : 1); - - /* Wait, or check for, available space. */ - if (filp->f_flags & O_NONBLOCK) { - ret = -EAGAIN; - if (PIPE_FREE(*inode) < free) - goto out; - } else { - while (PIPE_FREE(*inode) < free) { - PIPE_WAITING_WRITERS(*inode)++; - pipe_wait(inode); - PIPE_WAITING_WRITERS(*inode)--; - ret = -ERESTARTSYS; - if (signal_pending(current)) - goto out; - - if (!PIPE_READERS(*inode)) - goto sigpipe; + min = count; + if (min > PIPE_BUF) + min = 1; + down(PIPE_SEM(*inode)); + for (;;) { + int free; + if (!PIPE_READERS(*inode)) { + send_sig(SIGPIPE, current, 0); + if (!ret) ret = -EPIPE; + break; } - } - - /* Copy into available space. */ - ret = -EFAULT; - while (count > 0) { - int space; - char *pipebuf = PIPE_BASE(*inode) + PIPE_END(*inode); - ssize_t chars = PIPE_MAX_WCHUNK(*inode); - - if ((space = PIPE_FREE(*inode)) != 0) { + free = PIPE_FREE(*inode); + if (free >= min) { + /* transfer data */ + ssize_t chars = PIPE_MAX_WCHUNK(*inode); + char *pipebuf = PIPE_BASE(*inode) + PIPE_END(*inode); + /* Always wakeup, even if the copy fails. Otherwise + * we lock up (O_NONBLOCK-)readers that sleep due to + * syscall merging. + */ + do_wakeup = 1; if (chars > count) chars = count; - if (chars > space) - chars = space; + if (chars > free) + chars = free; - if (copy_from_user(pipebuf, buf, chars)) - goto out; + if (copy_from_user(pipebuf, buf, chars)) { + if (!ret) ret = -EFAULT; + break; + } - written += chars; + ret += chars; PIPE_LEN(*inode) += chars; count -= chars; buf += chars; - space = PIPE_FREE(*inode); + } + if (!count) + break; + if (PIPE_FREE(*inode) && ret) { + /* handle cyclic data buffers */ + min = 1; continue; } - - ret = written; - if (filp->f_flags & O_NONBLOCK) + if (filp->f_flags & O_NONBLOCK) { + if (!ret) ret = -EAGAIN; break; - - do { - /* - * Synchronous wake-up: it knows that this process - * is going to give up this CPU, so it doesn't have - * to do idle reschedules. - */ + } + if (signal_pending(current)) { + if (!ret) ret = -ERESTARTSYS; + break; + } + if (do_wakeup) { wake_up_interruptible_sync(PIPE_WAIT(*inode)); - kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN); - PIPE_WAITING_WRITERS(*inode)++; - pipe_wait(inode); - PIPE_WAITING_WRITERS(*inode)--; - if (signal_pending(current)) - goto out; - if (!PIPE_READERS(*inode)) - goto sigpipe; - } while (!PIPE_FREE(*inode)); - ret = -EFAULT; + kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN); + do_wakeup = 0; + } + PIPE_WAITING_WRITERS(*inode)++; + pipe_wait(inode); + PIPE_WAITING_WRITERS(*inode)--; } - - /* Signal readers asynchronously that there is more data. */ - wake_up_interruptible(PIPE_WAIT(*inode)); - kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN); - - inode->i_ctime = inode->i_mtime = CURRENT_TIME; - mark_inode_dirty(inode); - -out: up(PIPE_SEM(*inode)); -out_nolock: - if (written) - ret = written; + if (do_wakeup) { + wake_up_interruptible(PIPE_WAIT(*inode)); + kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN); + } + if (ret > 0) { + inode->i_ctime = inode->i_mtime = CURRENT_TIME; + mark_inode_dirty(inode); + } return ret; - -sigpipe: - if (written) - goto out; - up(PIPE_SEM(*inode)); - send_sig(SIGPIPE, current, 0); - return -EPIPE; } static ssize_t @@ -525,7 +486,7 @@ struct inode* pipe_new(struct inode* ino PIPE_BASE(*inode) = (char*) page; PIPE_START(*inode) = PIPE_LEN(*inode) = 0; PIPE_READERS(*inode) = PIPE_WRITERS(*inode) = 0; - PIPE_WAITING_READERS(*inode) = PIPE_WAITING_WRITERS(*inode) = 0; + PIPE_WAITING_WRITERS(*inode) = 0; PIPE_RCOUNTER(*inode) = PIPE_WCOUNTER(*inode) = 1; *PIPE_FASYNC_READERS(*inode) = *PIPE_FASYNC_WRITERS(*inode) = NULL; diff -urpN --exclude-from=/home/davej/.exclude bk-linus/include/linux/pipe_fs_i.h linux-2.5/include/linux/pipe_fs_i.h --- bk-linus/include/linux/pipe_fs_i.h 2002-08-20 13:15:58.000000000 +0100 +++ linux-2.5/include/linux/pipe_fs_i.h 2002-08-20 13:26:25.000000000 +0100 @@ -9,7 +9,6 @@ struct pipe_inode_info { unsigned int start; unsigned int readers; unsigned int writers; - unsigned int waiting_readers; unsigned int waiting_writers; unsigned int r_counter; unsigned int w_counter; @@ -28,7 +27,6 @@ struct pipe_inode_info { #define PIPE_LEN(inode) ((inode).i_pipe->len) #define PIPE_READERS(inode) ((inode).i_pipe->readers) #define PIPE_WRITERS(inode) ((inode).i_pipe->writers) -#define PIPE_WAITING_READERS(inode) ((inode).i_pipe->waiting_readers) #define PIPE_WAITING_WRITERS(inode) ((inode).i_pipe->waiting_writers) #define PIPE_RCOUNTER(inode) ((inode).i_pipe->r_counter) #define PIPE_WCOUNTER(inode) ((inode).i_pipe->w_counter)
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