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Date: Mon, 10 Feb 2020 09:25:11 -0800 From: Matthew Wilcox <willy@...radead.org> To: Qian Cai <cai@....pw> Cc: akpm@...ux-foundation.org, elver@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH -next] mm/filemap: fix a data race in filemap_fault() On Mon, Feb 10, 2020 at 12:00:29PM -0500, Qian Cai wrote: > @@ -2622,7 +2622,7 @@ void filemap_map_pages(struct vm_fault *vmf, > if (page->index >= max_idx) > goto unlock; > > - if (file->f_ra.mmap_miss > 0) > + if (data_race(file->f_ra.mmap_miss > 0)) > file->f_ra.mmap_miss--; How is this safe? Two threads can each see 1, and then both decrement the in-memory copy, causing it to end up at -1.
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