| Message ID | 20211025070015.553813-1-songyuanzheng@huawei.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [-next] mm/percpu: fix data-race with pcpu_nr_empty_pop_pages | expand |
On Mon, 25 Oct 2021, Yuanzheng Song wrote: > When reading the pcpu_nr_empty_pop_pages in pcpu_alloc() > and writing the pcpu_nr_empty_pop_pages in > pcpu_update_empty_pages() at the same time, > the data-race occurs. Looks like a use case for the atomic RMV instructions. > To fix this issue, use READ_ONCE() and WRITE_ONCE() to > read and write the pcpu_nr_empty_pop_pages. Never thought that READ_ONCE and WRITE_ONCE can fix races like this. Really? > diff --git a/mm/percpu.c b/mm/percpu.c > index 293009cc03ef..e8ef92e698ab 100644 > --- a/mm/percpu.c > +++ b/mm/percpu.c > @@ -574,7 +574,9 @@ static void pcpu_isolate_chunk(struct pcpu_chunk *chunk) > > if (!chunk->isolated) { > chunk->isolated = true; > - pcpu_nr_empty_pop_pages -= chunk->nr_empty_pop_pages; > + WRITE_ONCE(pcpu_nr_empty_pop_pages, > + READ_ONCE(pcpu_nr_empty_pop_pages) - > + chunk->nr_empty_pop_pages); atomic_sub()? > } > list_move(&chunk->list, &pcpu_chunk_lists[pcpu_to_depopulate_slot]); > } > @@ -585,7 +587,9 @@ static void pcpu_reintegrate_chunk(struct pcpu_chunk *chunk) > > if (chunk->isolated) { > chunk->isolated = false; > - pcpu_nr_empty_pop_pages += chunk->nr_empty_pop_pages; > + WRITE_ONCE(pcpu_nr_empty_pop_pages, > + READ_ONCE(pcpu_nr_empty_pop_pages) + > + chunk->nr_empty_pop_pages); atomic_add()?
Hello, On Mon, Oct 25, 2021 at 09:50:48AM +0200, Christoph Lameter wrote: > On Mon, 25 Oct 2021, Yuanzheng Song wrote: > > > When reading the pcpu_nr_empty_pop_pages in pcpu_alloc() > > and writing the pcpu_nr_empty_pop_pages in > > pcpu_update_empty_pages() at the same time, > > the data-race occurs. > > Looks like a use case for the atomic RMV instructions. > Yeah. I see 2 options. Switch the variable over to an atomic or we can move the read behind pcpu_lock. All the writes are already behind it othewise that would actually be problematic. In this particular case, reading a wrong # of empty pages isn't a big deal as eventually the background work will get scheduled. Thanks, Dennis > > To fix this issue, use READ_ONCE() and WRITE_ONCE() to > > read and write the pcpu_nr_empty_pop_pages. > > Never thought that READ_ONCE and WRITE_ONCE can fix races like > this. Really? > > > diff --git a/mm/percpu.c b/mm/percpu.c > > index 293009cc03ef..e8ef92e698ab 100644 > > --- a/mm/percpu.c > > +++ b/mm/percpu.c > > @@ -574,7 +574,9 @@ static void pcpu_isolate_chunk(struct pcpu_chunk *chunk) > > > > if (!chunk->isolated) { > > chunk->isolated = true; > > - pcpu_nr_empty_pop_pages -= chunk->nr_empty_pop_pages; > > + WRITE_ONCE(pcpu_nr_empty_pop_pages, > > + READ_ONCE(pcpu_nr_empty_pop_pages) - > > + chunk->nr_empty_pop_pages); > > atomic_sub()? > > > } > > list_move(&chunk->list, &pcpu_chunk_lists[pcpu_to_depopulate_slot]); > > } > > @@ -585,7 +587,9 @@ static void pcpu_reintegrate_chunk(struct pcpu_chunk *chunk) > > > > if (chunk->isolated) { > > chunk->isolated = false; > > - pcpu_nr_empty_pop_pages += chunk->nr_empty_pop_pages; > > + WRITE_ONCE(pcpu_nr_empty_pop_pages, > > + READ_ONCE(pcpu_nr_empty_pop_pages) + > > + chunk->nr_empty_pop_pages); > > atomic_add()? >
Hello, Thanks for the advice, Dennis Zhou and Christoph Lameter. I really appreciate it. I edited this patch by changing the pcpu_nr_empty_pop_pages to atomic_t variable. Here is the v2 patch: https://patchwork.kernel.org/project/linux-mm/patch/20211026084312.2138852-1-songyuanzheng@huawei.com/. Would you mind reviewing it again? Thanks, Yuanzheng Song -----Original Message----- From: Dennis Zhou [mailto:dennis@kernel.org] Sent: Tuesday, October 26, 2021 10:42 AM To: Christoph Lameter <cl@gentwo.de> Cc: songyuanzheng <songyuanzheng@huawei.com>; dennis@kernel.org; tj@kernel.org; akpm@linux-foundation.org; linux-mm@kvack.org; linux-kernel@vger.kernel.org Subject: Re: [PATCH -next] mm/percpu: fix data-race with pcpu_nr_empty_pop_pages Hello, On Mon, Oct 25, 2021 at 09:50:48AM +0200, Christoph Lameter wrote: > On Mon, 25 Oct 2021, Yuanzheng Song wrote: > > > When reading the pcpu_nr_empty_pop_pages in pcpu_alloc() and writing > > the pcpu_nr_empty_pop_pages in > > pcpu_update_empty_pages() at the same time, the data-race occurs. > > Looks like a use case for the atomic RMV instructions. > Yeah. I see 2 options. Switch the variable over to an atomic or we can move the read behind pcpu_lock. All the writes are already behind it othewise that would actually be problematic. In this particular case, reading a wrong # of empty pages isn't a big deal as eventually the background work will get scheduled. Thanks, Dennis > > To fix this issue, use READ_ONCE() and WRITE_ONCE() to read and > > write the pcpu_nr_empty_pop_pages. > > Never thought that READ_ONCE and WRITE_ONCE can fix races like this. > Really? > > > diff --git a/mm/percpu.c b/mm/percpu.c index > > 293009cc03ef..e8ef92e698ab 100644 > > --- a/mm/percpu.c > > +++ b/mm/percpu.c > > @@ -574,7 +574,9 @@ static void pcpu_isolate_chunk(struct pcpu_chunk > > *chunk) > > > > if (!chunk->isolated) { > > chunk->isolated = true; > > - pcpu_nr_empty_pop_pages -= chunk->nr_empty_pop_pages; > > + WRITE_ONCE(pcpu_nr_empty_pop_pages, > > + READ_ONCE(pcpu_nr_empty_pop_pages) - > > + chunk->nr_empty_pop_pages); > > atomic_sub()? > > > } > > list_move(&chunk->list, > > &pcpu_chunk_lists[pcpu_to_depopulate_slot]); > > } > > @@ -585,7 +587,9 @@ static void pcpu_reintegrate_chunk(struct > > pcpu_chunk *chunk) > > > > if (chunk->isolated) { > > chunk->isolated = false; > > - pcpu_nr_empty_pop_pages += chunk->nr_empty_pop_pages; > > + WRITE_ONCE(pcpu_nr_empty_pop_pages, > > + READ_ONCE(pcpu_nr_empty_pop_pages) + > > + chunk->nr_empty_pop_pages); > > atomic_add()? >
diff --git a/mm/percpu.c b/mm/percpu.c index 293009cc03ef..e8ef92e698ab 100644 --- a/mm/percpu.c +++ b/mm/percpu.c @@ -574,7 +574,9 @@ static void pcpu_isolate_chunk(struct pcpu_chunk *chunk) if (!chunk->isolated) { chunk->isolated = true; - pcpu_nr_empty_pop_pages -= chunk->nr_empty_pop_pages; + WRITE_ONCE(pcpu_nr_empty_pop_pages, + READ_ONCE(pcpu_nr_empty_pop_pages) - + chunk->nr_empty_pop_pages); } list_move(&chunk->list, &pcpu_chunk_lists[pcpu_to_depopulate_slot]); } @@ -585,7 +587,9 @@ static void pcpu_reintegrate_chunk(struct pcpu_chunk *chunk) if (chunk->isolated) { chunk->isolated = false; - pcpu_nr_empty_pop_pages += chunk->nr_empty_pop_pages; + WRITE_ONCE(pcpu_nr_empty_pop_pages, + READ_ONCE(pcpu_nr_empty_pop_pages) + + chunk->nr_empty_pop_pages); pcpu_chunk_relocate(chunk, -1); } } @@ -603,7 +607,8 @@ static inline void pcpu_update_empty_pages(struct pcpu_chunk *chunk, int nr) { chunk->nr_empty_pop_pages += nr; if (chunk != pcpu_reserved_chunk && !chunk->isolated) - pcpu_nr_empty_pop_pages += nr; + WRITE_ONCE(pcpu_nr_empty_pop_pages, + READ_ONCE(pcpu_nr_empty_pop_pages) + nr); } /* @@ -1874,7 +1879,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved, mutex_unlock(&pcpu_alloc_mutex); } - if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW) + if (READ_ONCE(pcpu_nr_empty_pop_pages) < PCPU_EMPTY_POP_PAGES_LOW) pcpu_schedule_balance_work(); /* clear the areas and return address relative to base address */ @@ -2062,7 +2067,7 @@ static void pcpu_balance_populated(void) pcpu_atomic_alloc_failed = false; } else { nr_to_pop = clamp(PCPU_EMPTY_POP_PAGES_HIGH - - pcpu_nr_empty_pop_pages, + READ_ONCE(pcpu_nr_empty_pop_pages), 0, PCPU_EMPTY_POP_PAGES_HIGH); } @@ -2163,7 +2168,8 @@ static void pcpu_reclaim_populated(void) break; /* reintegrate chunk to prevent atomic alloc failures */ - if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_HIGH) { + if (READ_ONCE(pcpu_nr_empty_pop_pages) < + PCPU_EMPTY_POP_PAGES_HIGH) { reintegrate = true; goto end_chunk; }
When reading the pcpu_nr_empty_pop_pages in pcpu_alloc() and writing the pcpu_nr_empty_pop_pages in pcpu_update_empty_pages() at the same time, the data-race occurs. =========== read-write to 0xffffffff882fdd4c of 4 bytes by task 9424 on cpu 0: pcpu_update_empty_pages pcpu_chunk_populated pcpu_balance_populated pcpu_balance_workfn process_one_work worker_thread kthread ret_from_fork read to 0xffffffff882fdd4c of 4 bytes by task 9386 on cpu 3: pcpu_alloc __alloc_percpu_gfp fib_nh_common_init fib_nh_init fib_create_info fib_table_insert fib_magic ...... sock_sendmsg_nosec sock_sendmsg __sys_sendto __do_sys_sendto __se_sys_sendto __x64_sys_sendto do_syscall_64 entry_SYSCALL_64_after_hwframe ============ The same problem will occur in these functions: pcpu_reclaim_populated(), pcpu_update_empty_pages(), pcpu_isolate_chunk(). To fix this issue, use READ_ONCE() and WRITE_ONCE() to read and write the pcpu_nr_empty_pop_pages. Signed-off-by: Yuanzheng Song <songyuanzheng@huawei.com> --- mm/percpu.c | 18 ++++++++++++------ 1 file changed, 12 insertions(+), 6 deletions(-)