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authorChristian Brauner <brauner@kernel.org>2023-10-25 12:14:37 +0200
committerChristian Brauner <brauner@kernel.org>2023-10-25 22:17:04 +0200
commit61d4fb0b349ec1b33119913c3b0bd109de30142c (patch)
tree35534a9e15ba9e75416befa4aac62387a3f095ec /fs/file.c
parentc04d905f6c7c41f137de7e4a9279e5c938eb19ef (diff)
file, i915: fix file reference for mmap_singleton()
Today we got a report at [1] for rcu stalls on the i915 testsuite in [2] due to the conversion of files to SLAB_TYPSSAFE_BY_RCU. Afaict, get_file_rcu() goes into an infinite loop trying to carefully verify that i915->gem.mmap_singleton hasn't changed - see the splat below. So I stared at this code to figure out what it actually does. It seems that the i915->gem.mmap_singleton pointer itself never had rcu semantics. The i915->gem.mmap_singleton is replaced in file->f_op->release::singleton_release(): static int singleton_release(struct inode *inode, struct file *file) { struct drm_i915_private *i915 = file->private_data; cmpxchg(&i915->gem.mmap_singleton, file, NULL); drm_dev_put(&i915->drm); return 0; } The cmpxchg() is ordered against a concurrent update of i915->gem.mmap_singleton from mmap_singleton(). IOW, when mmap_singleton() fails to get a reference on i915->gem.mmap_singleton: While mmap_singleton() does rcu_read_lock(); file = get_file_rcu(&i915->gem.mmap_singleton); rcu_read_unlock(); it allocates a new file via anon_inode_getfile() and does smp_store_mb(i915->gem.mmap_singleton, file); So, then what happens in the case of this bug is that at some point fput() is called and drops the file->f_count to zero leaving the pointer in i915->gem.mmap_singleton in tact. Now, there might be delays until file->f_op->release::singleton_release() is called and i915->gem.mmap_singleton is set to NULL. Say concurrently another task hits mmap_singleton() and does: rcu_read_lock(); file = get_file_rcu(&i915->gem.mmap_singleton); rcu_read_unlock(); When get_file_rcu() fails to get a reference via atomic_inc_not_zero() it will try the reload from i915->gem.mmap_singleton expecting it to be NULL, assuming it has comparable semantics as we expect in __fget_files_rcu(). But it hasn't so it reloads the same pointer again, trying the same atomic_inc_not_zero() again and doing so until file->f_op->release::singleton_release() of the old file has been called. So, in contrast to __fget_files_rcu() here we want to not retry when atomic_inc_not_zero() has failed. We only want to retry in case we managed to get a reference but the pointer did change on reload. <3> [511.395679] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks: <3> [511.395716] rcu: Tasks blocked on level-1 rcu_node (CPUs 0-9): P6238 <3> [511.395934] rcu: (detected by 16, t=65002 jiffies, g=123977, q=439 ncpus=20) <6> [511.395944] task:i915_selftest state:R running task stack:10568 pid:6238 tgid:6238 ppid:1001 flags:0x00004002 <6> [511.395962] Call Trace: <6> [511.395966] <TASK> <6> [511.395974] ? __schedule+0x3a8/0xd70 <6> [511.395995] ? asm_sysvec_apic_timer_interrupt+0x1a/0x20 <6> [511.396003] ? lockdep_hardirqs_on+0xc3/0x140 <6> [511.396013] ? asm_sysvec_apic_timer_interrupt+0x1a/0x20 <6> [511.396029] ? get_file_rcu+0x10/0x30 <6> [511.396039] ? get_file_rcu+0x10/0x30 <6> [511.396046] ? i915_gem_object_mmap+0xbc/0x450 [i915] <6> [511.396509] ? i915_gem_mmap+0x272/0x480 [i915] <6> [511.396903] ? mmap_region+0x253/0xb60 <6> [511.396925] ? do_mmap+0x334/0x5c0 <6> [511.396939] ? vm_mmap_pgoff+0x9f/0x1c0 <6> [511.396949] ? rcu_is_watching+0x11/0x50 <6> [511.396962] ? igt_mmap_offset+0xfc/0x110 [i915] <6> [511.397376] ? __igt_mmap+0xb3/0x570 [i915] <6> [511.397762] ? igt_mmap+0x11e/0x150 [i915] <6> [511.398139] ? __trace_bprintk+0x76/0x90 <6> [511.398156] ? __i915_subtests+0xbf/0x240 [i915] <6> [511.398586] ? __pfx___i915_live_setup+0x10/0x10 [i915] <6> [511.399001] ? __pfx___i915_live_teardown+0x10/0x10 [i915] <6> [511.399433] ? __run_selftests+0xbc/0x1a0 [i915] <6> [511.399875] ? i915_live_selftests+0x4b/0x90 [i915] <6> [511.400308] ? i915_pci_probe+0x106/0x200 [i915] <6> [511.400692] ? pci_device_probe+0x95/0x120 <6> [511.400704] ? really_probe+0x164/0x3c0 <6> [511.400715] ? __pfx___driver_attach+0x10/0x10 <6> [511.400722] ? __driver_probe_device+0x73/0x160 <6> [511.400731] ? driver_probe_device+0x19/0xa0 <6> [511.400741] ? __driver_attach+0xb6/0x180 <6> [511.400749] ? __pfx___driver_attach+0x10/0x10 <6> [511.400756] ? bus_for_each_dev+0x77/0xd0 <6> [511.400770] ? bus_add_driver+0x114/0x210 <6> [511.400781] ? driver_register+0x5b/0x110 <6> [511.400791] ? i915_init+0x23/0xc0 [i915] <6> [511.401153] ? __pfx_i915_init+0x10/0x10 [i915] <6> [511.401503] ? do_one_initcall+0x57/0x270 <6> [511.401515] ? rcu_is_watching+0x11/0x50 <6> [511.401521] ? kmalloc_trace+0xa3/0xb0 <6> [511.401532] ? do_init_module+0x5f/0x210 <6> [511.401544] ? load_module+0x1d00/0x1f60 <6> [511.401581] ? init_module_from_file+0x86/0xd0 <6> [511.401590] ? init_module_from_file+0x86/0xd0 <6> [511.401613] ? idempotent_init_module+0x17c/0x230 <6> [511.401639] ? __x64_sys_finit_module+0x56/0xb0 <6> [511.401650] ? do_syscall_64+0x3c/0x90 <6> [511.401659] ? entry_SYSCALL_64_after_hwframe+0x6e/0xd8 <6> [511.401684] </TASK> Link: [1]: https://lore.kernel.org/intel-gfx/SJ1PR11MB6129CB39EED831784C331BAFB9DEA@SJ1PR11MB6129.namprd11.prod.outlook.com Link: [2]: https://intel-gfx-ci.01.org/tree/linux-next/next-20231013/bat-dg2-11/igt@i915_selftest@live@mman.html#dmesg-warnings10963 Cc: Jann Horn <jannh@google.com>, Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: https://lore.kernel.org/r/20231025-formfrage-watscheln-84526cd3bd7d@brauner Signed-off-by: Christian Brauner <brauner@kernel.org>
Diffstat (limited to 'fs/file.c')
-rw-r--r--fs/file.c25
1 files changed, 25 insertions, 0 deletions
diff --git a/fs/file.c b/fs/file.c
index 1a475d7d636e..5fb0b146e79e 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -927,6 +927,31 @@ struct file *get_file_rcu(struct file __rcu **f)
}
EXPORT_SYMBOL_GPL(get_file_rcu);
+/**
+ * get_file_active - try go get a reference to a file
+ * @f: the file to get a reference on
+ *
+ * In contast to get_file_rcu() the pointer itself isn't part of the
+ * reference counting.
+ *
+ * This function should rarely have to be used and only by users who
+ * understand the implications of SLAB_TYPESAFE_BY_RCU. Try to avoid it.
+ *
+ * Return: Returns @f with the reference count increased or NULL.
+ */
+struct file *get_file_active(struct file **f)
+{
+ struct file __rcu *file;
+
+ rcu_read_lock();
+ file = __get_file_rcu(f);
+ rcu_read_unlock();
+ if (IS_ERR(file))
+ file = NULL;
+ return file;
+}
+EXPORT_SYMBOL_GPL(get_file_active);
+
static inline struct file *__fget_files_rcu(struct files_struct *files,
unsigned int fd, fmode_t mask)
{