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__unhash_process acquires the dcache_lock while holding the
tasklist_lock for writing. This can deadlock. While trying to fix that
race, I found further races with proc_dentry.
The attached patch (hopefully) fixes all these races.
Changes:
- fs/proc/base.c assumed that p->pid is reset to 0 during exit. This is
not the case anymore. I now look at the count of the pid structure for
PIDTYPE_PID.
- pid_delete_dentry and pid_revalidate removed. The implementation
didn't work, and noone complained.
- only d_add the new proc entry if the task is still valid: d_add+d_drop
is a race, if a lookup happens between both calls.
- add a new spinlock that protects proc_dentry.
What do you think? I don't like the new spinlock, but I couldn't find
another solution.
--
Manfred
--=_courier-4464-1049659198-0001-2
Content-Type: text/plain; name="patch-proc_dentry"; charset=iso-8859-1
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filename="patch-proc_dentry"
// $Header$
// Kernel Version:
// VERSION = 2
// PATCHLEVEL = 5
// SUBLEVEL = 66
// EXTRAVERSION =
--- 2.5/include/linux/sched.h 2003-03-17 22:43:40.000000000 +0100
+++ build-2.5/include/linux/sched.h 2003-04-04 23:45:06.000000000 +0200
@@ -428,6 +428,8 @@
u32 self_exec_id;
/* Protection of (de-)allocation: mm, files, fs, tty */
spinlock_t alloc_lock;
+/* Protection of proc_dentry: nesting proc_lock, dcache_lock, write_lock_irq(&tasklist_lock); */
+ spinlock_t proc_lock;
/* context-switch lock */
spinlock_t switch_lock;
--- 2.5/fs/proc/base.c 2003-03-17 22:44:13.000000000 +0100
+++ build-2.5/fs/proc/base.c 2003-04-05 00:55:25.000000000 +0200
@@ -794,17 +794,11 @@
/* dentry stuff */
-/*
- * Exceptional case: normally we are not allowed to unhash a busy
- * directory. In this case, however, we can do it - no aliasing problems
- * due to the way we treat inodes.
- */
-static int pid_revalidate(struct dentry * dentry, int flags)
+static int pid_alive(struct task_struct *p)
{
- if (proc_task(dentry->d_inode)->pid)
- return 1;
- d_drop(dentry);
- return 0;
+ BUG_ON(p->pids[PIDTYPE_PID].pidptr != &p->pids[PIDTYPE_PID].pid);
+
+ return atomic_read(&p->pids[PIDTYPE_PID].pid.count);
}
static int pid_fd_revalidate(struct dentry * dentry, int flags)
@@ -835,35 +829,25 @@
static void pid_base_iput(struct dentry *dentry, struct inode *inode)
{
struct task_struct *task = proc_task(inode);
- write_lock_irq(&tasklist_lock);
+ spin_lock(&task->proc_lock);
if (task->proc_dentry == dentry)
task->proc_dentry = NULL;
- write_unlock_irq(&tasklist_lock);
+ spin_unlock(&task->proc_lock);
iput(inode);
}
-static int pid_delete_dentry(struct dentry * dentry)
-{
- return proc_task(dentry->d_inode)->pid == 0;
-}
-
static struct dentry_operations pid_fd_dentry_operations =
{
.d_revalidate = pid_fd_revalidate,
- .d_delete = pid_delete_dentry,
};
static struct dentry_operations pid_dentry_operations =
{
- .d_revalidate = pid_revalidate,
- .d_delete = pid_delete_dentry,
};
static struct dentry_operations pid_base_dentry_operations =
{
- .d_revalidate = pid_revalidate,
.d_iput = pid_base_iput,
- .d_delete = pid_delete_dentry,
};
/* Lookups */
@@ -1088,6 +1072,60 @@
.follow_link = proc_self_follow_link,
};
+/**
+ * proc_pid_unhash - Unhash /proc/<pid> entry from the dcache.
+ * @p: task that should be flushed.
+ *
+ * Drops the /proc/<pid> dcache entry from the hash chains.
+ *
+ * As a special exception, dropping happens even if the directory
+ * is busy. Normally we are not allowed to unhash a busy
+ * directory. In this case, however, we can do it - no aliasing problems
+ * due to the way we treat inodes. (No idea what that means - manfred)
+ *
+ * Dropping /proc/<pid> entries and detach_pid must be synchroneous,
+ * otherwise e.g. /proc/<pid>/exe might point to the wrong executable,
+ * if the pid value is immediately reused. This is enforced by
+ * - caller must acquire spin_lock(p->proc_lock)
+ * - must be called before detach_pid()
+ * - proc_pid_lookup acquires proc_lock, and checks that
+ * the target is not dead by looking at the attach count
+ * of PIDTYPE_PID.
+ */
+
+struct dentry *proc_pid_unhash(struct task_struct *p)
+{
+ struct dentry *proc_dentry;
+
+ proc_dentry = p->proc_dentry;
+ if (proc_dentry != NULL) {
+
+ spin_lock(&dcache_lock);
+ if (!d_unhashed(proc_dentry)) {
+ dget_locked(proc_dentry);
+ __d_drop(proc_dentry);
+ } else
+ proc_dentry = NULL;
+ spin_unlock(&dcache_lock);
+ }
+ return proc_dentry;
+}
+
+/**
+ * proc_pid_flush - recover memory used by stale /proc/<pid>/x entries
+ * @proc_entry: directoy to prune.
+ *
+ * Shrink the /proc directory that was used by the just killed thread.
+ */
+
+void proc_pid_flush(struct dentry *proc_dentry)
+{
+ if(proc_dentry != NULL) {
+ shrink_dcache_parent(proc_dentry);
+ dput(proc_dentry);
+ }
+}
+
/* SMP-safe */
struct dentry *proc_pid_lookup(struct inode *dir, struct dentry * dentry)
{
@@ -1136,13 +1174,16 @@
inode->i_flags|=S_IMMUTABLE;
dentry->d_op = &pid_base_dentry_operations;
- d_add(dentry, inode);
- read_lock(&tasklist_lock);
- proc_task(dentry->d_inode)->proc_dentry = dentry;
- read_unlock(&tasklist_lock);
- if (!proc_task(dentry->d_inode)->pid)
- d_drop(dentry);
- return NULL;
+
+ spin_lock(&task->proc_lock);
+ if (pid_alive(task)) {
+ task->proc_dentry = dentry;
+ d_add(dentry, inode);
+ dentry = NULL;
+ }
+ spin_unlock(&task->proc_lock);
+ if (!dentry)
+ return NULL;
out:
return ERR_PTR(-ENOENT);
}
--- 2.5/kernel/exit.c 2003-03-17 22:44:43.000000000 +0100
+++ build-2.5/kernel/exit.c 2003-04-05 00:38:18.000000000 +0200
@@ -21,6 +21,7 @@
#include <linux/ptrace.h>
#include <linux/profile.h>
#include <linux/mount.h>
+#include <linux/proc_fs.h>
#include <asm/uaccess.h>
#include <asm/pgtable.h>
@@ -31,10 +32,8 @@
int getrusage(struct task_struct *, int, struct rusage *);
-static struct dentry * __unhash_process(struct task_struct *p)
+static void __unhash_process(struct task_struct *p)
{
- struct dentry *proc_dentry;
-
nr_threads--;
detach_pid(p, PIDTYPE_PID);
detach_pid(p, PIDTYPE_TGID);
@@ -46,34 +45,25 @@
}
REMOVE_LINKS(p);
- proc_dentry = p->proc_dentry;
- if (unlikely(proc_dentry != NULL)) {
- spin_lock(&dcache_lock);
- if (!d_unhashed(proc_dentry)) {
- dget_locked(proc_dentry);
- __d_drop(proc_dentry);
- } else
- proc_dentry = NULL;
- spin_unlock(&dcache_lock);
- }
- return proc_dentry;
}
void release_task(struct task_struct * p)
{
- struct dentry *proc_dentry;
task_t *leader;
+ struct dentry *proc_dentry;
BUG_ON(p->state < TASK_ZOMBIE);
atomic_dec(&p->user->processes);
+ spin_lock(&p->proc_lock);
+ proc_dentry = proc_pid_unhash(p);
write_lock_irq(&tasklist_lock);
if (unlikely(p->ptrace))
__ptrace_unlink(p);
BUG_ON(!list_empty(&p->ptrace_list) || !list_empty(&p->ptrace_children));
__exit_signal(p);
__exit_sighand(p);
- proc_dentry = __unhash_process(p);
+ __unhash_process(p);
/*
* If we are the last non-leader member of the thread
@@ -92,11 +82,8 @@
p->parent->cnswap += p->nswap + p->cnswap;
sched_exit(p);
write_unlock_irq(&tasklist_lock);
-
- if (unlikely(proc_dentry != NULL)) {
- shrink_dcache_parent(proc_dentry);
- dput(proc_dentry);
- }
+ spin_unlock(&p->proc_lock);
+ proc_pid_flush(proc_dentry);
release_thread(p);
put_task_struct(p);
}
@@ -106,15 +93,13 @@
void unhash_process(struct task_struct *p)
{
struct dentry *proc_dentry;
-
+ spin_lock(&p->proc_lock);
+ proc_pid_unhash(p);
write_lock_irq(&tasklist_lock);
- proc_dentry = __unhash_process(p);
+ __unhash_process(p);
write_unlock_irq(&tasklist_lock);
-
- if (unlikely(proc_dentry != NULL)) {
- shrink_dcache_parent(proc_dentry);
- dput(proc_dentry);
- }
+ spin_unlock(&p->proc_lock);
+ proc_pid_flush(proc_dentry);
}
/*
--- 2.5/include/linux/init_task.h 2003-03-17 22:43:40.000000000 +0100
+++ build-2.5/include/linux/init_task.h 2003-04-04 23:51:12.000000000 +0200
@@ -101,6 +101,7 @@
.blocked = {{0}}, \
.posix_timers = LIST_HEAD_INIT(tsk.posix_timers), \
.alloc_lock = SPIN_LOCK_UNLOCKED, \
+ .proc_lock = SPIN_LOCK_UNLOCKED, \
.switch_lock = SPIN_LOCK_UNLOCKED, \
.journal_info = NULL, \
}
--- 2.5/fs/exec.c 2003-04-05 00:40:05.000000000 +0200
+++ build-2.5/fs/exec.c 2003-04-05 00:40:47.000000000 +0200
@@ -529,30 +529,6 @@
return 0;
}
-static struct dentry *clean_proc_dentry(struct task_struct *p)
-{
- struct dentry *proc_dentry = p->proc_dentry;
-
- if (proc_dentry) {
- spin_lock(&dcache_lock);
- if (!d_unhashed(proc_dentry)) {
- dget_locked(proc_dentry);
- __d_drop(proc_dentry);
- } else
- proc_dentry = NULL;
- spin_unlock(&dcache_lock);
- }
- return proc_dentry;
-}
-
-static inline void put_proc_dentry(struct dentry *dentry)
-{
- if (dentry) {
- shrink_dcache_parent(dentry);
- dput(dentry);
- }
-}
-
/*
* This function makes sure the current process has its own signal table,
* so that flush_signal_handlers can later reset the handlers without
@@ -660,9 +636,11 @@
while (leader->state != TASK_ZOMBIE)
yield();
+ task_lock(leader);
+ task_lock(current);
+ proc_dentry1 = proc_pid_unhash(current);
+ proc_dentry2 = proc_pid_unhash(leader);
write_lock_irq(&tasklist_lock);
- proc_dentry1 = clean_proc_dentry(current);
- proc_dentry2 = clean_proc_dentry(leader);
if (leader->tgid != current->tgid)
BUG();
@@ -702,9 +680,10 @@
state = leader->state;
write_unlock_irq(&tasklist_lock);
-
- put_proc_dentry(proc_dentry1);
- put_proc_dentry(proc_dentry2);
+ task_unlock(current);
+ task_unlock(leader);
+ proc_pid_flush(proc_dentry1);
+ proc_pid_flush(proc_dentry2);
if (state != TASK_ZOMBIE)
BUG();
--=_courier-4464-1049659198-0001-2--