Re: [PATCH] mm: Remove pointless might_sleep() in remove_vm_area().

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Andrey Ryabinin wrote:
> On 03/24/2017 01:53 PM, Tetsuo Handa wrote:
> > Commit 5803ed292e63a1bf ("mm: mark all calls into the vmalloc subsystem
> > as potentially sleeping") added might_sleep() to remove_vm_area() from
> > vfree(), and is causing
> > 
> > [    2.616064] BUG: sleeping function called from invalid context at mm/vmalloc.c:1480
> > [    2.616125] in_atomic(): 1, irqs_disabled(): 0, pid: 341, name: plymouthd
> > [    2.616156] 2 locks held by plymouthd/341:
> > [    2.616158]  #0:  (drm_global_mutex){+.+.+.}, at: [<ffffffffc01c274b>] drm_release+0x3b/0x3b0 [drm]
> > [    2.616256]  #1:  (&(&tfile->lock)->rlock){+.+...}, at: [<ffffffffc0173038>] ttm_object_file_release+0x28/0x90 [ttm]
> > [    2.616270] CPU: 2 PID: 341 Comm: plymouthd Not tainted 4.11.0-0.rc3.git0.1.kmallocwd.fc25.x86_64+debug #1
> > [    2.616271] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/02/2015
> > [    2.616273] Call Trace:
> > [    2.616281]  dump_stack+0x86/0xc3
> > [    2.616285]  ___might_sleep+0x17d/0x250
> > [    2.616289]  __might_sleep+0x4a/0x80
> > [    2.616293]  remove_vm_area+0x22/0x90
> > [    2.616296]  __vunmap+0x2e/0x110
> > [    2.616299]  vfree+0x42/0x90
> > [    2.616304]  kvfree+0x2c/0x40
> > [    2.616312]  drm_ht_remove+0x1a/0x30 [drm]
> > [    2.616317]  ttm_object_file_release+0x50/0x90 [ttm]
> > [    2.616324]  vmw_postclose+0x47/0x60 [vmwgfx]
> > [    2.616331]  drm_release+0x290/0x3b0 [drm]
> > [    2.616338]  __fput+0xf8/0x210
> > [    2.616342]  ____fput+0xe/0x10
> > [    2.616345]  task_work_run+0x85/0xc0
> > [    2.616351]  exit_to_usermode_loop+0xb4/0xc0
> > [    2.616355]  do_syscall_64+0x185/0x1f0
> > [    2.616359]  entry_SYSCALL64_slow_path+0x25/0x25
> > 
> > warning.
> > 
> > But commit f9e09977671b618a ("mm: turn vmap_purge_lock into a mutex") did
> > not make vfree() potentially sleeping because try_purge_vmap_area_lazy()
> > is still using mutex_trylock(). Thus, this is a false positive warning.
> > 
> 
> Commit f9e09977671b618a did not made vfree() sleeping.
> Commit 763b218ddfa "mm: add preempt points into __purge_vmap_area_lazy()"
> did this, thus it's not a false positive.
> 
> 
> > ___might_sleep() via cond_resched_lock() in __purge_vmap_area_lazy() from
> > try_purge_vmap_area_lazy() from free_vmap_area_noflush() from
> > free_unmap_vmap_area() from remove_vm_area() which might trigger same
> > false positive warning is remaining. But so far we haven't heard about
> > warning from that path.
> 
> And why that would be a false positive?
> 

#define cond_resched_lock(lock) ({                              \
	___might_sleep(__FILE__, __LINE__, PREEMPT_LOCK_OFFSET);\
	__cond_resched_lock(lock);                              \
	})

cond_resched_lock() calls ___might_sleep() even when
__cond_resched_lock() will not call preempt_schedule_common()
because should_resched() returns false due to preemption counter
being already elevated by holding &(&tfile->lock)->rlock spinlock.

If should_resched() is known to return false, calling
___might_sleep() from cond_resched_lock() is a false positive.

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