Re: [PATCH -next] mm/util: annotate an intentional data race

[Dennis Zhou <dennis@kernel.org>]

On Thu, Jan 30, 2020 at 09:56:49AM -0500, Qian Cai wrote:
> "vm_committed_as.count" could be accessed concurrently as reported by
> KCSAN,
> 
>  read to 0xffffffff923164f8 of 8 bytes by task 1268 on cpu 38:
>   __vm_enough_memory+0x43/0x280 mm/util.c:801
>   mmap_region+0x1b2/0xb90 mm/mmap.c:1726
>   do_mmap+0x45c/0x700
>   vm_mmap_pgoff+0xc0/0x130
>   vm_mmap+0x71/0x90
>   elf_map+0xa1/0x1b0
>   load_elf_binary+0x9de/0x2180
>   search_binary_handler+0xd8/0x2b0
>   __do_execve_file+0xb61/0x1080
>   __x64_sys_execve+0x5f/0x70
>   do_syscall_64+0x91/0xb47
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
>  write to 0xffffffff923164f8 of 8 bytes by task 1265 on cpu 41:
>   percpu_counter_add_batch+0x83/0xd0 lib/percpu_counter.c:91
>   exit_mmap+0x178/0x220 include/linux/mman.h:68
>   mmput+0x10e/0x270
>   flush_old_exec+0x572/0xfe0
>   load_elf_binary+0x467/0x2180
>   search_binary_handler+0xd8/0x2b0
>   __do_execve_file+0xb61/0x1080
>   __x64_sys_execve+0x5f/0x70
>   do_syscall_64+0x91/0xb47
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> The warning is almost impossible to trigger according to the commit
> 82f71ae4a2b8 ("mm: catch memory commitment underflow") but leave it for
> now to catch any possible unbalanced vm_unacct_memory() in the future.
> Since only the read is operating as lockless, mark it as an intentional
> data race using the data_race() macro.
> 
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>  mm/util.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 988d11e6c17c..528d2c710771 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -798,8 +798,8 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
>  {
>  	long allowed;
>  
> -	VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
> -			-(s64)vm_committed_as_batch * num_online_cpus(),
> +	VM_WARN_ONCE(data_race(percpu_counter_read(&vm_committed_as) <
> +			-(s64)vm_committed_as_batch * num_online_cpus()),
>  			"memory commitment underflow");
>  
>  	vm_acct_memory(pages);
> -- 
> 2.21.0 (Apple Git-122.2)
> 

Acked-by: Dennis Zhou <dennis@kernel.org>

Thanks,
Dennis