{"id":2227437,"url":"http://patchwork.ozlabs.org/api/patches/2227437/?format=json","web_url":"http://patchwork.ozlabs.org/project/linux-ext4/patch/20260423173700.GA2187084@chcpu16/","project":{"id":8,"url":"http://patchwork.ozlabs.org/api/projects/8/?format=json","name":"Linux ext4 filesystem development","link_name":"linux-ext4","list_id":"linux-ext4.vger.kernel.org","list_email":"linux-ext4@vger.kernel.org","web_url":null,"scm_url":null,"webscm_url":null,"list_archive_url":"","list_archive_url_format":"","commit_url_format":""},"msgid":"<20260423173700.GA2187084@chcpu16>","list_archive_url":null,"date":"2026-04-23T17:37:16","name":"[BUG] ext4: KCSAN: lockless i_es_all_nr reads in es_shrinker_info","commit_ref":null,"pull_url":null,"state":"new","archived":false,"hash":"36f03a33f6c68130a49b37bfbb3789c1b16af3c5","submitter":{"id":91564,"url":"http://patchwork.ozlabs.org/api/people/91564/?format=json","name":"Shuhao Fu","email":"sfual@cse.ust.hk"},"delegate":null,"mbox":"http://patchwork.ozlabs.org/project/linux-ext4/patch/20260423173700.GA2187084@chcpu16/mbox/","series":[{"id":501229,"url":"http://patchwork.ozlabs.org/api/series/501229/?format=json","web_url":"http://patchwork.ozlabs.org/project/linux-ext4/list/?series=501229","date":"2026-04-23T17:37:16","name":"[BUG] ext4: KCSAN: lockless i_es_all_nr reads in es_shrinker_info","version":1,"mbox":"http://patchwork.ozlabs.org/series/501229/mbox/"}],"comments":"http://patchwork.ozlabs.org/api/patches/2227437/comments/","check":"pending","checks":"http://patchwork.ozlabs.org/api/patches/2227437/checks/","tags":{},"related":[],"headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linux-ext4@vger.kernel.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","patchwork-incoming@ozlabs.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=fail reason=\"signature verification failed\" (1024-bit key;\n unprotected) header.d=cse.ust.hk header.i=@cse.ust.hk header.a=rsa-sha256\n header.s=cseusthk header.b=lePdqm4q;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=ozlabs.org\n (client-ip=2404:9400:2221:ea00::3; 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charset=us-ascii","Content-Disposition":"inline","X-Env-From":"sfual","X-Spam-Status":"No, score=0.0 required=5.0 tests=ARC_SIGNED,ARC_VALID,\n\tDKIM_INVALID,DKIM_SIGNED,DMARC_NONE,HEADER_FROM_DIFFERENT_DOMAINS,\n\tMAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=disabled\n\tversion=4.0.1","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on gandalf.ozlabs.org"},"content":"Hi,\n\nReading /proc/fs/ext4//es_shrinker_info can overlap with extent-status\nupdates and trigger KCSAN reports on the per-inode ES counters (I saw this on\ni_es_all_nr; i_es_shk_nr is read the same way in this proc path). From what I\ncan see, the user-visible impact appears limited to stale/inconsistent procfs\nstats output (I do not have evidence of corruption or crash from this path).\n\nI reproduced this on a local KCSAN-instrumented tree based on linux commit\nd8a9a4b11a13, using an x86_64 QEMU workload with userspace reader/writer loops.\nTo increase the race window, I added small debug-only hooks in my local tree:\nafter the writer updates the counter, it briefly delays and records which inode\nit just touched; the proc reader then samples that inode's counters during the\ns_es_list walk. I also wrapped the i_es_all_nr load in a local helper\next4_es_shrinker_read_all_nr() so the read-side stack has a stable symbol;\nupstream reads happen directly in ext4_seq_es_shrinker_info_show().\n\nWith that setup, KCSAN prints the following summary line (naming the two\nracing functions):\n\n BUG: KCSAN: data-race in ext4_es_init_extent / ext4_es_shrinker_read_all_nr\n\nThe first clean hit in my local log was:\n\n read to 0xffff917cc15222c8 of 4 bytes by task 107 on cpu 0:\n ext4_es_shrinker_read_all_nr+0x26/0x50\n ext4_es_kcsan_probe_hot_inode+0x2b9/0x400\n ext4_seq_es_shrinker_info_show+0x9b/0xd40\n ...\n __x64_sys_sendfile64+0xc2/0x100\n do_syscall_64+0x13f/0x3c0\n\n write (reordered) to 0xffff917cc15222c8 of 4 bytes by task 108 on cpu 2:\n ext4_es_init_extent+0x6aa/0xa00\n __es_insert_extent+0x477/0xaa0\n ...\n ext4_do_fallocate+0x127/0x310\n __x64_sys_fallocate+0x75/0xb0\n\nI then saw the same pair again later in the same run (for example around\n129.529391 and 129.579938), still on the same 4-byte address.\n\nIt looks like i_es_all_nr and i_es_shk_nr are documented as protected by\ni_es_lock, and writers update them under i_es_lock, but\next4_seq_es_shrinker_info_show() reads them while walking the list under\ns_es_lock (the list lock), not i_es_lock.\n\nThe reproducer shape from normal userspace APIs is one reader loop running\ncat /proc/fs/ext4//es_shrinker_info while a writer loop runs fallocate,\nbuffered writes, punch-hole, and truncate on the same filesystem.\n\nSince this appears to be an observational procfs stats path, would you prefer\nmarking these loads with data_race(...) so the intentionally approximate reads\nare explicit and this path stops generating repeated KCSAN warnings?\n\nThe rough change I had in mind is:\n\n+++ b/fs/ext4/extents_status.c\n@@\n int ext4_seq_es_shrinker_info_show(struct seq_file *seq, void *v)\n {\n \t...\n \tlist_for_each_entry(ei, &sbi->s_es_list, i_es_list) {\n \t\tinode_cnt++;\n \t\tei_all_nr = data_race(ei->i_es_all_nr);\n \t\tei_shk_nr = data_race(ei->i_es_shk_nr);\n \t\t...\n }\n\nIf this direction is preferred, I can send a formal patch.\n\nThanks,\nShuhao","diff":"diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c\nindex ... .. ...\n--- a/fs/ext4/extents_status.c\n","prefixes":["BUG"]}