Messages in this thread Patch in this message | | | Date | Thu, 20 May 2010 17:00:04 +1000 | From | Nick Piggin <> | Subject | [patch 2/3] ipc: use shifts to extract seq/idx |
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All the ipc ids and sequences are signed integers, so power of 2 sequence multiplier does not work so well. Convert it to use shifts, which improves generated code particularly in ipc_lock/ipc_lock_check fast paths.
Index: linux-2.6/ipc/util.c =================================================================== --- linux-2.6.orig/ipc/util.c +++ linux-2.6/ipc/util.c @@ -123,7 +123,7 @@ void ipc_init_ids(struct ipc_ids *ids) ids->in_use = 0; ids->seq = 0; { - int seq_limit = INT_MAX/SEQ_MULTIPLIER; + int seq_limit = INT_MAX >> SEQ_SHIFT; if (seq_limit > USHORT_MAX) ids->seq_max = USHORT_MAX; else Index: linux-2.6/ipc/util.h =================================================================== --- linux-2.6.orig/ipc/util.h +++ linux-2.6/ipc/util.h @@ -13,9 +13,11 @@ #include <linux/unistd.h> #include <linux/err.h> -#define IPCMNI_MAX 32768 /* <= MAX_INT absolute limit for ipc arrays */ +/* IPCMNI_MAX should be <= MAX_INT, absolute limit for ipc arrays */ +#define IPCMNI_MAX_SHIFT 15 +#define IPCMNI_MAX (1 << IPCMNI_MAX_SHIFT) -#define SEQ_MULTIPLIER (IPCMNI_MAX) +#define SEQ_SHIFT IPCMNI_MAX_SHIFT void sem_init (void); void msg_init (void); @@ -93,7 +95,7 @@ void __init ipc_init_proc_interface(cons #define IPC_MSG_IDS 1 #define IPC_SHM_IDS 2 -#define ipcid_to_idx(id) ((id) % SEQ_MULTIPLIER) +#define ipcid_to_idx(id) ((id) & ((1UL << SEQ_SHIFT) - 1)) /* must be called with ids->rw_mutex acquired for writing */ int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int); @@ -146,7 +148,7 @@ extern void recompute_msgmni(struct ipc_ static inline int ipc_buildid(int id, int seq) { - return SEQ_MULTIPLIER * seq + id; + return (seq << SEQ_SHIFT) + id; } /* @@ -154,7 +156,7 @@ static inline int ipc_buildid(int id, in */ static inline int ipc_checkid(struct kern_ipc_perm *ipcp, int uid) { - if (uid / SEQ_MULTIPLIER != ipcp->seq) + if ((uid >> SEQ_SHIFT) != ipcp->seq) return 1; return 0; }
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