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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 31 Jul 2025 04:30:58.9897 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: ae3812c1-78f8-47a1-3ed4-08ddcfeb0c86 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB5PEPF00014B9D.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB9PR08MB6652 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250730_213207_241528_FDF95434 X-CRM114-Status: GOOD ( 14.73 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 30/07/25 11:59 pm, Ryan Roberts wrote: > On 30/07/2025 18:00, Catalin Marinas wrote: >> On Wed, Jul 23, 2025 at 09:48:27PM +0530, Dev Jain wrote: > [...] > >>> + * mmap_write_lock/unlock in T1 be called CS (the critical section). >>> + * >>> + * Claim: The CS of T1 will never operate on a freed PMD table. >>> + * >>> + * Proof: >>> + * >>> + * Case 1: The static branch is visible to T2. >>> + * >>> + * Case 1 (a): T1 acquires the lock before T2 can. >>> + * T2 will block until T1 drops the lock, so pmd_free() will only be >>> + * executed after T1 exits CS. >> This assumes that there is some ordering between unlock and pmd_free() >> (e.g. some poisoning of the old page). The unlock only gives us release >> semantics, not acquire. It just happens that we have an atomic >> dec-and-test down the __free_pages() path but I'm not convinced we >> should rely on it unless free_pages() has clear semantics on ordering >> related to prior memory writes. > I can understand how pmd_free() could be re-ordered before the unlock, but > surely it can't be reorded before the lock? I need to go unlearn everything I > thought I understood about locking if that's the case... > You are correct, what Catalin is saying is that my reasoning has a hole. There is no obvious ordering between unlock and free(), but mmap_write_unlock() will happen before mmap_read_lock() ... (i) mmap_read_lock() will happen before pmd_free() ... (ii) which lets us conclude that mmap_write_unlock() will happen before pmd_free(). A more rigorous way to write this would be (for Case 1): T2 T1 pmd_clear() 5. cmpxchg(enable static branch) 1. while (!cmpxchg(check if lock not taken in write mode)); 6. smp_mb() 2. smp_mb(); 7. while (!cmpxchg(check if lock not taken)) 3. smp_mb(); 8. smp_mb() 4. cmpxchg(release lock) CS instructions pmd_free() 9. smp_mb() 10. cmpxchg (release lock) where: (1,2) = mmap_read_lock(), (3,4) = mmap_read_unlock(), (5,6) = static_branch_enable(), (7,8) = mmap_write_lock(), (9, 10) = mmap_write_unlock(). For Case 1(a), 7 succeeds before 1 can. So, 1 will block until 10 completes, so 10 is observed before 1, and 1 is observed before pmd_free() due to the barriers. The associativity follows.