'\" t
.\"     Title: sb_start_write
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: January 2017
.\"    Manual: The Linux VFS
.\"    Source: Kernel Hackers Manual 4.8.15
.\"  Language: English
.\"
.TH "SB_START_WRITE" "9" "January 2017" "Kernel Hackers Manual 4\&.8\&." "The Linux VFS"
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.SH "NAME"
sb_start_write \- get write access to a superblock
.SH "SYNOPSIS"
.HP \w'void\ sb_start_write('u
.BI "void sb_start_write(struct\ super_block\ *\ " "sb" ");"
.SH "ARGUMENTS"
.PP
\fIsb\fR
.RS 4
the super we write to
.RE
.SH "DESCRIPTION"
.PP
When a process wants to write data or metadata to a file system (i\&.e\&. dirty a page or an inode), it should embed the operation in a
\fBsb_start_write\fR
\-
\fBsb_end_write\fR
pair to get exclusion against file system freezing\&. This function increments number of writers preventing freezing\&. If the file system is already frozen, the function waits until the file system is thawed\&.
.PP
Since freeze protection behaves as a lock, users have to preserve ordering of freeze protection and other filesystem locks\&. Generally, freeze protection should be the outermost lock\&. In particular, we have:
.PP
sb_start_write \-> i_mutex (write path, truncate, directory ops, \&.\&.\&.) \-> s_umount (freeze_super, thaw_super)
.SH "COPYRIGHT"
.br