Commit 3077a260 authored by Paul Jackson's avatar Paul Jackson Committed by Linus Torvalds
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[PATCH] cpuset release ABBA deadlock fix

Fix possible cpuset_sem ABBA deadlock if 'notify_on_release' set.

For a particular usage pattern, creating and destroying cpusets fairly
frequently using notify_on_release, on a very large system, this deadlock
can be seen every few days.  If you are not using the cpuset
notify_on_release feature, you will never see this deadlock.

The existing code, on task exit (or cpuset deletion) did:

  get cpuset_sem
  if cpuset marked notify_on_release and is ready to release:
    compute cpuset path relative to /dev/cpuset mount point
    call_usermodehelper() forks /sbin/cpuset_release_agent with path
  drop cpuset_sem

Unfortunately, the fork in call_usermodehelper can allocate memory, and
allocating memory can require cpuset_sem, if the mems_generation values
changed in the interim.  This results in an ABBA deadlock, trying to obtain
cpuset_sem when it is already held by the current task.

To fix this, I put the cpuset path (which must be computed while holding
cpuset_sem) in a temporary buffer, to be used in the call_usermodehelper
call of /sbin/cpuset_release_agent only _after_ dropping cpuset_sem.

So the new logic is:

  get cpuset_sem
  if cpuset marked notify_on_release and is ready to release:
    compute cpuset path relative to /dev/cpuset mount point
    stash path in kmalloc'd buffer
  drop cpuset_sem
  call_usermodehelper() forks /sbin/cpuset_release_agent with path
  free path

The sharp eyed reader might notice that this patch does not contain any
calls to kmalloc.  The existing code in the check_for_release() routine was
already kmalloc'ing a buffer to hold the cpuset path.  In the old code, it
just held the buffer for a few lines, over the cpuset_release_agent() call
that in turn invoked call_usermodehelper().  In the new code, with the
application of this patch, it returns that buffer via the new char
**ppathbuf parameter, for later use and freeing in cpuset_release_agent(),
which is called after cpuset_sem is dropped.  Whereas the old code has just
one call to cpuset_release_agent(), right in the check_for_release()
routine, the new code has three calls to cpuset_release_agent(), from the
various places that a cpuset can be released.

This patch has been build and booted on SN2, and passed a stress test that
previously hit the deadlock within a few seconds.
Signed-off-by: default avatarPaul Jackson <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent a242b44d
......@@ -398,21 +398,31 @@ static int cpuset_path(const struct cpuset *cs, char *buf, int buflen)
* to continue to serve a useful existence. Next time it's released,
* we will get notified again, if it still has 'notify_on_release' set.
* Note final arg to call_usermodehelper() is 0 - that means
* don't wait. Since we are holding the global cpuset_sem here,
* and we are asking another thread (started from keventd) to rmdir a
* cpuset, we can't wait - or we'd deadlock with the removing thread
* on cpuset_sem.
* The final arg to call_usermodehelper() is 0, which means don't
* wait. The separate /sbin/cpuset_release_agent task is forked by
* call_usermodehelper(), then control in this thread returns here,
* without waiting for the release agent task. We don't bother to
* wait because the caller of this routine has no use for the exit
* status of the /sbin/cpuset_release_agent task, so no sense holding
* our caller up for that.
* The simple act of forking that task might require more memory,
* which might need cpuset_sem. So this routine must be called while
* cpuset_sem is not held, to avoid a possible deadlock. See also
* comments for check_for_release(), below.
static int cpuset_release_agent(char *cpuset_str)
static void cpuset_release_agent(const char *pathbuf)
char *argv[3], *envp[3];
int i;
if (!pathbuf)
i = 0;
argv[i++] = "/sbin/cpuset_release_agent";
argv[i++] = cpuset_str;
argv[i++] = (char *)pathbuf;
argv[i] = NULL;
i = 0;
......@@ -421,17 +431,29 @@ static int cpuset_release_agent(char *cpuset_str)
envp[i++] = "PATH=/sbin:/bin:/usr/sbin:/usr/bin";
envp[i] = NULL;
return call_usermodehelper(argv[0], argv, envp, 0);
call_usermodehelper(argv[0], argv, envp, 0);
* Either cs->count of using tasks transitioned to zero, or the
* cs->children list of child cpusets just became empty. If this
* cs is notify_on_release() and now both the user count is zero and
* the list of children is empty, send notice to user land.
* the list of children is empty, prepare cpuset path in a kmalloc'd
* buffer, to be returned via ppathbuf, so that the caller can invoke
* cpuset_release_agent() with it later on, once cpuset_sem is dropped.
* Call here with cpuset_sem held.
* This check_for_release() routine is responsible for kmalloc'ing
* pathbuf. The above cpuset_release_agent() is responsible for
* kfree'ing pathbuf. The caller of these routines is responsible
* for providing a pathbuf pointer, initialized to NULL, then
* calling check_for_release() with cpuset_sem held and the address
* of the pathbuf pointer, then dropping cpuset_sem, then calling
* cpuset_release_agent() with pathbuf, as set by check_for_release().
static void check_for_release(struct cpuset *cs)
static void check_for_release(struct cpuset *cs, char **ppathbuf)
if (notify_on_release(cs) && atomic_read(&cs->count) == 0 &&
list_empty(&cs->children)) {
......@@ -441,10 +463,9 @@ static void check_for_release(struct cpuset *cs)
if (!buf)
if (cpuset_path(cs, buf, PAGE_SIZE) < 0)
goto out;
*ppathbuf = buf;
......@@ -727,14 +748,14 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs, char *buf)
return 0;
static int attach_task(struct cpuset *cs, char *buf)
static int attach_task(struct cpuset *cs, char *pidbuf, char **ppathbuf)
pid_t pid;
struct task_struct *tsk;
struct cpuset *oldcs;
cpumask_t cpus;
if (sscanf(buf, "%d", &pid) != 1)
if (sscanf(pidbuf, "%d", &pid) != 1)
return -EIO;
if (cpus_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed))
return -ENOSPC;
......@@ -777,7 +798,7 @@ static int attach_task(struct cpuset *cs, char *buf)
if (atomic_dec_and_test(&oldcs->count))
check_for_release(oldcs, ppathbuf);
return 0;
......@@ -801,6 +822,7 @@ static ssize_t cpuset_common_file_write(struct file *file, const char __user *us
struct cftype *cft = __d_cft(file->f_dentry);
cpuset_filetype_t type = cft->private;
char *buffer;
char *pathbuf = NULL;
int retval = 0;
/* Crude upper limit on largest legitimate cpulist user might write. */
......@@ -841,7 +863,7 @@ static ssize_t cpuset_common_file_write(struct file *file, const char __user *us
retval = update_flag(CS_NOTIFY_ON_RELEASE, cs, buffer);
retval = attach_task(cs, buffer);
retval = attach_task(cs, buffer, &pathbuf);
retval = -EINVAL;
......@@ -852,6 +874,7 @@ static ssize_t cpuset_common_file_write(struct file *file, const char __user *us
retval = nbytes;
return retval;
......@@ -1357,6 +1380,7 @@ static int cpuset_rmdir(struct inode *unused_dir, struct dentry *dentry)
struct cpuset *cs = dentry->d_fsdata;
struct dentry *d;
struct cpuset *parent;
char *pathbuf = NULL;
/* the vfs holds both inode->i_sem already */
......@@ -1376,7 +1400,7 @@ static int cpuset_rmdir(struct inode *unused_dir, struct dentry *dentry)
list_del(&cs->sibling); /* delete my sibling from parent->children */
if (list_empty(&parent->children))
check_for_release(parent, &pathbuf);
d = dget(cs->dentry);
cs->dentry = NULL;
......@@ -1384,6 +1408,7 @@ static int cpuset_rmdir(struct inode *unused_dir, struct dentry *dentry)
return 0;
......@@ -1483,10 +1508,13 @@ void cpuset_exit(struct task_struct *tsk)
if (notify_on_release(cs)) {
char *pathbuf = NULL;
if (atomic_dec_and_test(&cs->count))
check_for_release(cs, &pathbuf);
} else {
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