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    Subject[PATCH v6 12/24] erofs: introduce tagged pointer
    Date
    Currently kernel has scattered tagged pointer usages
    hacked by hand in plain code, without a unique and
    portable functionset to highlight the tagged pointer
    itself and wrap these hacked code in order to clean up
    all over meaningless magic masks.

    This patch introduces simple generic methods to fold
    tags into a pointer integer. Currently it supports
    the last n bits of the pointer for tags, which can be
    selected by users.

    In addition, it will also be used for the upcoming EROFS
    filesystem, which heavily uses tagged pointer pproach
    to reduce extra memory allocation.

    Link: https://en.wikipedia.org/wiki/Tagged_pointer

    Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
    ---
    fs/erofs/tagptr.h | 110 ++++++++++++++++++++++++++++++++++++++++++++++
    1 file changed, 110 insertions(+)
    create mode 100644 fs/erofs/tagptr.h

    diff --git a/fs/erofs/tagptr.h b/fs/erofs/tagptr.h
    new file mode 100644
    index 000000000000..a72897c86744
    --- /dev/null
    +++ b/fs/erofs/tagptr.h
    @@ -0,0 +1,110 @@
    +/* SPDX-License-Identifier: GPL-2.0-only */
    +/*
    + * A tagged pointer implementation
    + *
    + * Copyright (C) 2018 Gao Xiang <gaoxiang25@huawei.com>
    + */
    +#ifndef __EROFS_FS_TAGPTR_H
    +#define __EROFS_FS_TAGPTR_H
    +
    +#include <linux/types.h>
    +#include <linux/build_bug.h>
    +
    +/*
    + * the name of tagged pointer types are tagptr{1, 2, 3...}_t
    + * avoid directly using the internal structs __tagptr{1, 2, 3...}
    + */
    +#define __MAKE_TAGPTR(n) \
    +typedef struct __tagptr##n { \
    + uintptr_t v; \
    +} tagptr##n##_t;
    +
    +__MAKE_TAGPTR(1)
    +__MAKE_TAGPTR(2)
    +__MAKE_TAGPTR(3)
    +__MAKE_TAGPTR(4)
    +
    +#undef __MAKE_TAGPTR
    +
    +extern void __compiletime_error("bad tagptr tags")
    + __bad_tagptr_tags(void);
    +
    +extern void __compiletime_error("bad tagptr type")
    + __bad_tagptr_type(void);
    +
    +/* fix the broken usage of "#define tagptr2_t tagptr3_t" by users */
    +#define __tagptr_mask_1(ptr, n) \
    + __builtin_types_compatible_p(typeof(ptr), struct __tagptr##n) ? \
    + (1UL << (n)) - 1 :
    +
    +#define __tagptr_mask(ptr) (\
    + __tagptr_mask_1(ptr, 1) ( \
    + __tagptr_mask_1(ptr, 2) ( \
    + __tagptr_mask_1(ptr, 3) ( \
    + __tagptr_mask_1(ptr, 4) ( \
    + __bad_tagptr_type(), 0)))))
    +
    +/* generate a tagged pointer from a raw value */
    +#define tagptr_init(type, val) \
    + ((typeof(type)){ .v = (uintptr_t)(val) })
    +
    +/*
    + * directly cast a tagged pointer to the native pointer type, which
    + * could be used for backward compatibility of existing code.
    + */
    +#define tagptr_cast_ptr(tptr) ((void *)(tptr).v)
    +
    +/* encode tagged pointers */
    +#define tagptr_fold(type, ptr, _tags) ({ \
    + const typeof(_tags) tags = (_tags); \
    + if (__builtin_constant_p(tags) && (tags & ~__tagptr_mask(type))) \
    + __bad_tagptr_tags(); \
    +tagptr_init(type, (uintptr_t)(ptr) | tags); })
    +
    +/* decode tagged pointers */
    +#define tagptr_unfold_ptr(tptr) \
    + ((void *)((tptr).v & ~__tagptr_mask(tptr)))
    +
    +#define tagptr_unfold_tags(tptr) \
    + ((tptr).v & __tagptr_mask(tptr))
    +
    +/* operations for the tagger pointer */
    +#define tagptr_eq(_tptr1, _tptr2) ({ \
    + typeof(_tptr1) tptr1 = (_tptr1); \
    + typeof(_tptr2) tptr2 = (_tptr2); \
    + (void)(&tptr1 == &tptr2); \
    +(tptr1).v == (tptr2).v; })
    +
    +/* lock-free CAS operation */
    +#define tagptr_cmpxchg(_ptptr, _o, _n) ({ \
    + typeof(_ptptr) ptptr = (_ptptr); \
    + typeof(_o) o = (_o); \
    + typeof(_n) n = (_n); \
    + (void)(&o == &n); \
    + (void)(&o == ptptr); \
    +tagptr_init(o, cmpxchg(&ptptr->v, o.v, n.v)); })
    +
    +/* wrap WRITE_ONCE if atomic update is needed */
    +#define tagptr_replace_tags(_ptptr, tags) ({ \
    + typeof(_ptptr) ptptr = (_ptptr); \
    + *ptptr = tagptr_fold(*ptptr, tagptr_unfold_ptr(*ptptr), tags); \
    +*ptptr; })
    +
    +#define tagptr_set_tags(_ptptr, _tags) ({ \
    + typeof(_ptptr) ptptr = (_ptptr); \
    + const typeof(_tags) tags = (_tags); \
    + if (__builtin_constant_p(tags) && (tags & ~__tagptr_mask(*ptptr))) \
    + __bad_tagptr_tags(); \
    + ptptr->v |= tags; \
    +*ptptr; })
    +
    +#define tagptr_clear_tags(_ptptr, _tags) ({ \
    + typeof(_ptptr) ptptr = (_ptptr); \
    + const typeof(_tags) tags = (_tags); \
    + if (__builtin_constant_p(tags) && (tags & ~__tagptr_mask(*ptptr))) \
    + __bad_tagptr_tags(); \
    + ptptr->v &= ~tags; \
    +*ptptr; })
    +
    +#endif /* __EROFS_FS_TAGPTR_H */
    +
    --
    2.17.1
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