linux-kernel - Re: [PATCH] XArray: Make xa_lock

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Date:   Mon, 23 Oct 2023 21:26:31 -0400
From:   Waiman Long <longman@...hat.com>
To:     Stanislaw Gruszka <stanislaw.gruszka@...ux.intel.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH] XArray: Make xa_lock_init macro


On 10/23/23 04:49, Stanislaw Gruszka wrote:
> On Mon, Oct 02, 2023 at 10:25:35AM +0200, Stanislaw Gruszka wrote:
>> Make xa_init_flags() macro to avoid false positive lockdep splats.
>
> Friendly ping. The subject should be changed to mention xa_init_flags(),
> but anything else should be done here to get it apply ?
>
> Regards
> Stanislaw
>
>
>> When spin_lock_init() is used inside initialization function (like
>> in xa_init_flags()) which can be called many times, lockdep assign
>> the same key to different locks.
>>
>> For example this splat is seen with intel_vpu driver which uses
>> two xarrays and has two separate xa_init_flags() calls:
>>
>> [ 1139.148679] WARNING: inconsistent lock state
>> [ 1139.152941] 6.6.0-hardening.1+ #2 Tainted: G           OE
>> [ 1139.158758] --------------------------------
>> [ 1139.163024] inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.
>> [ 1139.169018] kworker/10:1/109 [HC1[1]:SC0[0]:HE0:SE1] takes:
>> [ 1139.174576] ffff888137237150 (&xa->xa_lock#18){?.+.}-{2:2}, at: ivpu_mmu_user_context_mark_invalid+0x1c/0x80 [intel_vpu]
>> [ 1139.185438] {HARDIRQ-ON-W} state was registered at:
>> [ 1139.190305]   lock_acquire+0x1a3/0x4a0
>> [ 1139.194055]   _raw_spin_lock+0x2c/0x40
>> [ 1139.197800]   ivpu_submit_ioctl+0xf0b/0x3520 [intel_vpu]
>> [ 1139.203114]   drm_ioctl_kernel+0x201/0x3f0 [drm]
>> [ 1139.207791]   drm_ioctl+0x47d/0xa20 [drm]
>> [ 1139.211846]   __x64_sys_ioctl+0x12e/0x1a0
>> [ 1139.215849]   do_syscall_64+0x59/0x90
>> [ 1139.219509]   entry_SYSCALL_64_after_hwframe+0x6e/0xd8
>> [ 1139.224636] irq event stamp: 45500
>> [ 1139.228037] hardirqs last  enabled at (45499): [<ffffffff92ef0314>] _raw_spin_unlock_irq+0x24/0x50
>> [ 1139.236961] hardirqs last disabled at (45500): [<ffffffff92eadf8f>] common_interrupt+0xf/0x90
>> [ 1139.245457] softirqs last  enabled at (44956): [<ffffffff92ef3430>] __do_softirq+0x4c0/0x712
>> [ 1139.253862] softirqs last disabled at (44461): [<ffffffff907df310>] irq_exit_rcu+0xa0/0xd0
>> [ 1139.262098]
>>                 other info that might help us debug this:
>> [ 1139.268604]  Possible unsafe locking scenario:
>>
>> [ 1139.274505]        CPU0
>> [ 1139.276955]        ----
>> [ 1139.279403]   lock(&xa->xa_lock#18);
>> [ 1139.282978]   <Interrupt>
>> [ 1139.285601]     lock(&xa->xa_lock#18);
>> [ 1139.289345]
>>                  *** DEADLOCK ***
>>
>> Lockdep falsely identified xa_lock from two different xarrays as the same
>> lock and report deadlock. More detailed description of the problem
>> is provided in commit c21f11d182c2 ("drm: fix drmm_mutex_init()")
>>
>> Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@...ux.intel.com>
>> ---
>>   include/linux/xarray.h | 17 +++++++----------
>>   1 file changed, 7 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
>> index cb571dfcf4b1..409d9d739ee9 100644
>> --- a/include/linux/xarray.h
>> +++ b/include/linux/xarray.h
>> @@ -375,12 +375,12 @@ void xa_destroy(struct xarray *);
>>    *
>>    * Context: Any context.
>>    */
>> -static inline void xa_init_flags(struct xarray *xa, gfp_t flags)
>> -{
>> -	spin_lock_init(&xa->xa_lock);
>> -	xa->xa_flags = flags;
>> -	xa->xa_head = NULL;
>> -}
>> +#define xa_init_flags(_xa, _flags)	\
>> +do {					\
>> +	spin_lock_init(&(_xa)->xa_lock);\
>> +	(_xa)->xa_flags = (_flags);	\
>> +	(_xa)->xa_head = NULL;		\
>> +} while (0)
>>   
>>   /**
>>    * xa_init() - Initialise an empty XArray.
>> @@ -390,10 +390,7 @@ static inline void xa_init_flags(struct xarray *xa, gfp_t flags)
>>    *
>>    * Context: Any context.
>>    */
>> -static inline void xa_init(struct xarray *xa)
>> -{
>> -	xa_init_flags(xa, 0);
>> -}
>> +#define xa_init(xa) xa_init_flags(xa, 0)
>>   
>>   /**
>>    * xa_empty() - Determine if an array has any present entries.
>> -- 
>> 2.25.1
>>
LGTM. However, it is up to Matthew to take it or not as he is the XArray 
maintainer.

Acked-by: Waiman Long <longman@...hat.com>