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The basic rules are simple: * stores to dentry->d_flags are OK under dentry->d_lock. * stores to dentry->d_flags are OK in the dentry constructor, before becomes potentially visible to other threads. Unfortunately, there's a couple of exceptions to that, and that's where the headache comes from. Main PITA comes from d_set_d_op(); that primitive sets ->d_op of dentry and adjusts the flags that correspond to presence of individual methods. It's very easy to misuse; existing uses _are_ safe, but proof of correctness is brittle. Use in __d_alloc() is safe (we are within a constructor), but we might as well precalculate the initial value of ->d_flags when we set the default ->d_op for given superblock and set ->d_flags directly instead of messing with that helper. The reasons why other uses are safe are bloody convoluted; I'm not going to reproduce it here. See https://lore.kernel.org/all/20250224010624.GT1977892@ZenIV/ for gory details, if you care. The critical part is using d_set_d_op() only just prior to d_splice_alias(), which makes a combination of d_splice_alias() with setting ->d_op, etc. a natural replacement primitive. Better yet, if we go that way, it's easy to take setting ->d_op and modifying ->d_flags under ->d_lock, which eliminates the headache as far as ->d_flags exclusion rules are concerned. Other exceptions are minor and easy to deal with. What this series does: * d_set_d_op() is no longer available; new primitive (d_splice_alias_ops()) is provided, equivalent to combination of d_set_d_op() and d_splice_alias(). * new field of struct super_block - ->s_d_flags. Default value of ->d_flags to be used when allocating dentries on this filesystem. * new primitive for setting ->s_d_op: set_default_d_op(). Replaces stores to ->s_d_op at mount time. All in-tree filesystems converted; out-of-tree ones will get caught by compiler (->s_d_op is renamed, so stores to it will be caught). ->s_d_flags is set by the same primitive to match the ->s_d_op. * a lot of filesystems had ->s_d_op->d_delete equal to always_delete_dentry; that is equivalent to setting DCACHE_DONTCACHE in ->d_flags, so such filesystems can bloody well set that bit in ->s_d_flags and drop ->d_delete() from dentry_operations. In quite a few cases that results in empty dentry_operations, which means that we can get rid of those. * kill simple_dentry_operations - not needed anymore. * massage d_alloc_parallel() to get rid of the other exception wrt ->d_flags stores - we can set DCACHE_PAR_LOOKUP as soon as we allocate the new dentry; no need to delay that until we commit to using the sucker. As the result, ->d_flags stores are all either under ->d_lock or done before the dentry becomes visible in any shared data structures. -----BEGIN PGP SIGNATURE----- iHUEABYIAB0WIQQqUNBr3gm4hGXdBJlZ7Krx/gZQ6wUCaIQ/tQAKCRBZ7Krx/gZQ 66AhAQDgQ+S224x5YevNXc9mDoGUBMF4OG0n0fIla9rfdL4I6wEAqpOWMNDcVPCZ GwYOvJ9YuqNdz+MyprAI18Yza4GOmgs= =rTYB -----END PGP SIGNATURE----- Merge tag 'pull-dcache' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs Pull dentry d_flags updates from Al Viro: "The current exclusion rules for dentry->d_flags stores are rather unpleasant. The basic rules are simple: - stores to dentry->d_flags are OK under dentry->d_lock - stores to dentry->d_flags are OK in the dentry constructor, before becomes potentially visible to other threads Unfortunately, there's a couple of exceptions to that, and that's where the headache comes from. The main PITA comes from d_set_d_op(); that primitive sets ->d_op of dentry and adjusts the flags that correspond to presence of individual methods. It's very easy to misuse; existing uses _are_ safe, but proof of correctness is brittle. Use in __d_alloc() is safe (we are within a constructor), but we might as well precalculate the initial value of 'd_flags' when we set the default ->d_op for given superblock and set 'd_flags' directly instead of messing with that helper. The reasons why other uses are safe are bloody convoluted; I'm not going to reproduce it here. See [1] for gory details, if you care. The critical part is using d_set_d_op() only just prior to d_splice_alias(), which makes a combination of d_splice_alias() with setting ->d_op, etc a natural replacement primitive. Better yet, if we go that way, it's easy to take setting ->d_op and modifying 'd_flags' under ->d_lock, which eliminates the headache as far as 'd_flags' exclusion rules are concerned. Other exceptions are minor and easy to deal with. What this series does: - d_set_d_op() is no longer available; instead a new primitive (d_splice_alias_ops()) is provided, equivalent to combination of d_set_d_op() and d_splice_alias(). - new field of struct super_block - 's_d_flags'. This sets the default value of 'd_flags' to be used when allocating dentries on this filesystem. - new primitive for setting 's_d_op': set_default_d_op(). This replaces stores to 's_d_op' at mount time. All in-tree filesystems converted; out-of-tree ones will get caught by the compiler ('s_d_op' is renamed, so stores to it will be caught). 's_d_flags' is set by the same primitive to match the 's_d_op'. - a lot of filesystems had sb->s_d_op->d_delete equal to always_delete_dentry; that is equivalent to setting DCACHE_DONTCACHE in 'd_flags', so such filesystems can bloody well set that bit in 's_d_flags' and drop 'd_delete()' from dentry_operations. In quite a few cases that results in empty dentry_operations, which means that we can get rid of those. - kill simple_dentry_operations - not needed anymore - massage d_alloc_parallel() to get rid of the other exception wrt 'd_flags' stores - we can set DCACHE_PAR_LOOKUP as soon as we allocate the new dentry; no need to delay that until we commit to using the sucker. As the result, 'd_flags' stores are all either under ->d_lock or done before the dentry becomes visible in any shared data structures" Link: https://lore.kernel.org/all/20250224010624.GT1977892@ZenIV/ [1] * tag 'pull-dcache' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs: (21 commits) configfs: use DCACHE_DONTCACHE debugfs: use DCACHE_DONTCACHE efivarfs: use DCACHE_DONTCACHE instead of always_delete_dentry() 9p: don't bother with always_delete_dentry ramfs, hugetlbfs, mqueue: set DCACHE_DONTCACHE kill simple_dentry_operations devpts, sunrpc, hostfs: don't bother with ->d_op shmem: no dentry retention past the refcount reaching zero d_alloc_parallel(): set DCACHE_PAR_LOOKUP earlier make d_set_d_op() static simple_lookup(): just set DCACHE_DONTCACHE tracefs: Add d_delete to remove negative dentries set_default_d_op(): calculate the matching value for ->d_flags correct the set of flags forbidden at d_set_d_op() time split d_flags calculation out of d_set_d_op() new helper: set_default_d_op() fuse: no need for special dentry_operations for root dentry switch procfs from d_set_d_op() to d_splice_alias_ops() new helper: d_splice_alias_ops() procfs: kill ->proc_dops ... |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.