Is RCU not safe enough to protect dcache hlist?
Hi, all: I am studying the dcache in VFS recently . I found that hlist of dcache is already protected by RCU in __d_lookup.Is it necessary for the function -- d_lookup using sequence lock to protect the hlist again? struct dentry * d_lookup(struct dentry * parent, struct qstr * name) { struct dentry * dentry = NULL; unsigned long seq; do { seq = read_seqbegin(&rename_lock); dentry = __d_lookup(parent, name); if (dentry) break; } while (read_seqretry(&rename_lock, seq)); return dentry; } Thanks --- Rock Lee
On Mon, 16 Mar 2015 10:06:37 +0800, Lee said:
I am studying the dcache in VFS recently . I found that hlist of dcache is already protected by RCU in __d_lookup.Is it necessary for the function -- d_lookup using sequence lock to protect the hlist again?
RCU is useful when you can tolerate a bit of latency in other users finding out about updated values and/or slightly stale values. That's not an option for d_lookup(), which has to check the most recent value in order to avoid race conditions.
On 2015/3/16 14:02, Valdis.Kletnieks@vt.edu wrote:
On Mon, 16 Mar 2015 10:06:37 +0800, Lee said:
I am studying the dcache in VFS recently . I found that hlist of dcache is already protected by RCU in __d_lookup.Is it necessary for the function -- d_lookup using sequence lock to protect the hlist again?
RCU is useful when you can tolerate a bit of latency in other users finding out about updated values and/or slightly stale values. That's not an option for d_lookup(), which has to check the most recent value in order to avoid race conditions.
In __d_lookup(), one hlist is protected well with RCU. But this is not enough for the dentry_hashtable, so d_lookup() uses some mechanism to protect the dentry_hashtable, in this condition, the mechanism is sequence lock :-)
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Valdis.Kletnieks＠vt.edu