Hey,
2013/2/18 Fengguang Wu <fengguang...@intel.com>:
> Greetings,
>
> I got the below warning and the first bad commit is
>
> commit 11eb5a2ce9655ee552f705f2da6b8f96f466c0d4
> Author: Maarten Lankhorst <maarten.lankho...@canonical.com>
> Date: Wed Nov 7 11:53:51 2012 +0100
>
> mutex: add support for reservation style locks
>
> GPU's do operations that commonly involve many buffers. Those buffers
> can be shared across contexts/processes, exist in different memory
> domains (for example VRAM vs system memory), and so on. And with
> PRIME / dmabuf, they can even be shared across devices. So there are
> a handful of situations where the driver needs to wait for buffers to
> become ready. If you think about this in terms of waiting on a buffer
> mutex for it to become available, this presents a problem because
> there is no way to guarantee that buffers appear in a execbuf/batch in
> the same order in all contexts. That is directly under control of
> userspace, and a result of the sequence of GL calls that an application
> makes. Which results in the potential for deadlock. The problem gets
> more complex when you consider that the kernel may need to migrate the
> buffer(s) into VRAM before the GPU operates on the buffer(s), which
> may in turn require evicting some other buffers (and you don't want to
> evict other buffers which are already queued up to the GPU), but for a
> simplified understanding of the problem you can ignore this.
>
> The algorithm that TTM came up with for dealing with this problem is
> quite simple. For each group of buffers (execbuf) that need to be
> locked, the caller would be assigned a unique reservation_id, from a
> global counter. In case of deadlock in the process of locking all the
> buffers associated with a execbuf, the one with the lowest
> reservation_id wins, and the one with the higher reservation_id
> unlocks all of the buffers that it has already locked, and then tries
> again.
>
> How it is used:
> ---------------
>
> A very simplified version:
>
> int lock_execbuf(execbuf)
> {
> struct buf *res_buf = NULL;
>
> /* acquiring locks, before queuing up to GPU: */
> seqno = assign_global_seqno();
>
> retry:
> for (buf in execbuf->buffers) {
> if (buf == res_buf) {
> res_buf = NULL;
> continue;
> }
> ret = mutex_reserve_lock(&buf->lock, seqno);
> if (ret < 0)
> goto err;
> }
>
> /* now everything is good to go, submit job to GPU: */
> ...
>
> return 0;
>
> err:
> for (all buf2 before buf in execbuf->buffers)
> mutex_unreserve_unlock(&buf2->lock);
> if (res_buf)
> mutex_unreserve_unlock(&res_buf->lock);
>
> if (ret == -EAGAIN) {
> /* we lost out in a seqno race, lock and retry.. */
> mutex_reserve_lock_slow(&buf->lock, seqno);
> res_buf = buf;
> goto retry;
> }
>
> return ret;
> }
>
> int unlock_execbuf(execbuf)
> {
> /* when GPU is finished; */
> for (buf in execbuf->buffers)
> mutex_unreserve_unlock(&buf->lock);
> }
>
> Functions:
> ----------
>
> mutex_reserve_lock, and mutex_reserve_lock_interruptible:
> Lock a buffer with a reservation_id set. reservation_id must not be
> set to 0, since this is a special value that means no reservation_id.
>
> Normally if reservation_id is not set, or is older than the
> reservation_id that's currently set on the mutex, the behavior will
> be to wait normally. However, if the reservation_id is newer than
> the current reservation_id, -EAGAIN will be returned.
>
> These functions will return -EDEADLK instead of -EAGAIN if
> reservation_id is the same as the reservation_id that's attempted to
> lock the mutex with, since in that case you presumably attempted to
> lock the same lock twice.
>
> mutex_reserve_lock_slow and mutex_reserve_lock_intr_slow:
> Similar to mutex_reserve_lock, except it won't backoff with -EAGAIN.
> This is useful when mutex_reserve_lock failed with -EAGAIN, and you
> unreserved all buffers so no deadlock can occur.
>
> mutex_unreserve_unlock:
> Unlock a buffer reserved with one of the mutex_reserve_*lock* calls.
>
> Missing at the moment, maybe TODO?
> * Check if lockdep warns if you unlock a lock that other locks were
> nested
> to.
> - spin_lock(m);
> spin_lock_nest_lock(a, m);
> spin_unlock(m);
> spin_unlock(a);
> It would be nice if this would give a splat on spin_unlock(m),
> I'm not 100% sure if it does right now, though..
> * In the *_slow calls, maybe add a check to ensure no other locks of
> the same
> lockdep class are nested in the lock we have to nest to?
> - This is making sure that mutex_unreserve_unlock have been called on
> all other locks.
>
> Design:
> I chose for ticket_mutex to encapsulate struct mutex, so the extra
> memory usage and
> atomic set on init will only happen when you deliberately create a
> ticket lock.
>
> Since the mutexes are mostly meant to protect buffer object
> serialization in ttm, not
> much contention is expected. I could be slightly smarter with wakeups,
> but this would
> be at the expense at adding a field to struct mutex_waiter. This would
> add
> overhead to all cases where normal mutexes are used, and ticket_mutexes
> are less
> performance sensitive anyway since they only protect buffer objects. As
> a result
> I chose not to do this.
>
> I needed this in kernel/mutex.c because of the extensions to
> __lock_common, which are
> hopefully optimized away for all normal paths.
>
> It is not illegal to use mutex_lock and mutex_unlock on ticket mutexes.
> This will work,
> as long you don't mix lock/unlock calls. This is useful if you want to
> lock only a single
> buffer.
>
> All the mutex_reserve calls are nested into another lock. The idea is
> that the
> seqno ticket you use functions as a lockdep class you have locked too.
> This will
> prevent lockdep false positives on locking 2 objects of the same class.
> It's allowed
> because they're nested to the seqno ticket. There are extensive tests
> for this in the
> patch that introduces locking tests for reservation objects and
> reservation tickets.
>
> Changes since RFC patch v1:
> - Updated to use atomic_long instead of atomic, since the reservation_id
> was a long.
> - added mutex_reserve_lock_slow and mutex_reserve_lock_intr_slow
> - removed mutex_locked_set_reservation_id (or w/e it was called)
> Changes since RFC patch v2:
> - remove use of __mutex_lock_retval_arg, add warnings when using wrong
> combination of
> mutex_(,reserve_)lock/unlock.
>
> Signed-off-by: Maarten Lankhorst <maarten.lankho...@canonical.com>
>
> [ 0.000000]
> [ 0.000000] ------------[ cut here ]------------
> [ 0.000000] WARNING: at /c/kernel-tests/src/tip/kernel/mutex.c:386
> __mutex_lock_common+0x5a9/0x870()
> [ 0.000000] Hardware name: Bochs
> [ 0.000000] Modules linked in:
> [ 0.000000] Pid: 0, comm: swapper/0 Not tainted 3.8.0-rc7-00071-g11eb5a2
> #180
> [ 0.000000] Call Trace:
> [ 0.000000] [<ffffffff81047102>] warn_slowpath_common+0xb2/0x120
> [ 0.000000] [<ffffffff81047195>] warn_slowpath_null+0x25/0x30
> [ 0.000000] [<ffffffff814ebdd9>] __mutex_lock_common+0x5a9/0x870
> [ 0.000000] [<ffffffff81a66ef8>] ? rcu_cpu_notify+0xa8/0x451
> [ 0.000000] [<ffffffff810bcb8f>] ? trace_hardirqs_off_caller+0xaf/0x120
> [ 0.000000] [<ffffffff81a66ef8>] ? rcu_cpu_notify+0xa8/0x451
> [ 0.000000] [<ffffffff810be38c>] ? lockdep_init_map+0xfc/0x230
> [ 0.000000] [<ffffffff814ec621>] mutex_lock_nested+0x61/0x80
> [ 0.000000] [<ffffffff810bcc1d>] ? trace_hardirqs_off+0x1d/0x30
> [ 0.000000] [<ffffffff81a66ef8>] rcu_cpu_notify+0xa8/0x451
> [ 0.000000] [<ffffffff814ecc31>] ? mutex_unlock+0x11/0x20
> [ 0.000000] [<ffffffff81a3f952>] rcu_init+0x3b3/0x408
> [ 0.000000] [<ffffffff81a21e0c>] start_kernel+0x34a/0x744
> [ 0.000000] [<ffffffff81a216e6>] ? repair_env_string+0x81/0x81
> [ 0.000000] [<ffffffff81a21120>] ? early_idt_handlers+0x120/0x120
> [ 0.000000] [<ffffffff81a212fd>] x86_64_start_reservations+0x185/0x190
> [ 0.000000] [<ffffffff81a214a8>] x86_64_start_kernel+0x1a0/0x1b6
> [ 0.000000] ---[ end trace 8e966724b1809892 ]---
Weird, that code path should not be hit from __mutex_lock_common from
mutex_lock_nested, I'll create a patch with some tests to make sure
that lib/locking-selftests.c will perform tests on common mutexes to
ensure that code path is not hit, and this bug will not happen again.
Can you change __mutex_lock_common from inline to __always_inline and
check if that gets rid of the warning?
~Maarten
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